1. Field of the Invention
The present invention relates to a liquid ejection head, for ejecting liquid droplets to print on a print medium, and to a printing apparatus, which employs the liquid ejection head.
2. Description of the Related Arts
Inkjet printing apparatuses that have so rapidly become popular are equipped with liquid ejection heads that, while being conveyed in the scanning direction, eject ink droplets and print on the print media. Advantages afforded by these inkjet printing apparatuses include the ease of design and the production of compact units and the ease of use when performing color printing.
Also, recently, in response to an increasing demand for inkjet printing apparatuses that provide better image quality, a present trend is for the number of heaters used as heat generation elements to be increased in order to perform faster printing using smaller liquid droplets. With this arrangement, since the current that flows to and through the heaters is increased, on the whole, a reduction in the power consumed wastefully by wiring is required. As one method for reducing the power consumed by wiring for carrying current to the heaters, resistance at the heaters may be increased, so that a large quantity of heat can be generated and applied to ink, even when only a small current is flowing across the heaters. Thus, the heaters may be formed by a thin film to reduce their cross-sectional sizes, and to increase their resistance to the transmission of electricity. However, in a case wherein, to heat ink, one heater is employed for each nozzle, there is a limitation on the acceptable reduction in the thickness of the film of the heaters, even when the heaters are formed by the thin film to increase their resistance. Therefore, an arrangement, such as that disclosed for an inkjet printing apparatus in Japanese Patent Laid-Open No. 2004-1488, wherein two or more heaters, arranged within a print head and electrically connected in series, may be employed in the above described case. In this instance, it is preferable that the interval between the heaters be as small as possible, so that the thermal energy generated by the heaters can be efficiently transferred to ink.
As a printing method to be employed by an inkjet printing apparatus, a dot density control method has been proposed whereby, for the expression of a half tone, the number of print dots in a unit area is controlled using print dots of a predetermined size. According to this method, a print head that includes nozzles having different ejection port diameters, and thus ejects ink droplets having different sizes, is employed as means for controlling the number of print dots. Then, print dots are formed by using small ink droplets for the bright portion and the intermediate portion of an image, while print dots are formed by using large ink droplets for the half tone portion and the dark portion of the image. A printing method performed in this way is proposed. As an example print head that includes nozzles that enable the ejection of ink droplets having different sizes, an arrangement wherein nozzles are alternately arranged in a zigzag pattern is a generally known means used to increase nozzle density and to provide a high resolution nozzle array. In Japanese Patent Laid-Open No. 2005-1379, a printing apparatus is disclosed that has a nozzle array obtained by arranging, in a zigzag pattern, nozzles that enable the ejection of ink droplets having different dot diameters.
By the way, a problem included in this inkjet printing apparatus is that inside a print head, cavitation occurs as bubbles collapse. To resolve this internal print head cavitation problem, an inkjet printing apparatus and a print head are disclosed, for example, in Japanese Patent Laid-Open No. H04-10941 (1992).
According to Japanese Patent Laid-Open No. H04-10941 (1992), this print head is formed such that the bubble generated during the ejection of ink droplets communicates with the air. Therefore, when the size of the bubble is reduced, the bubble is dispersed into the air, and does not remain within the print head. Thus, cavitation that occurs as the bubble collapses can be avoided, and damage to areas in the vicinities of the heaters can be prevented.
However, when a print head that includes a plurality of nozzles having ejection ports of different diameters is employed to cope with a request for faster printing or for higher image quality, as described above, it becomes difficult for the system that permits bubbles to communicate with the air to prevent the cavitation that occurs during the collapse of bubbles.
Even for the above described print head, wherein a plurality of nozzles that provide different ink ejection quantities are formed in a single substrate, the distance from the surface of a substrate to an ejection port must be the same for all the ejection ports, because of manufacturing requirements for producing the print head; however, the sizes of bubbles formed inside the print head vary, depending on the sizes of ejected ink droplets. And if a print head is designed to permit bubbles to communicate with the air, when an ink droplet is being ejected from a nozzle that provides a large ink ejection quantity, there is a difficulty that the bubble communicates with the air inside a nozzle for providing a small quantity of ink for ejection. Therefore, it is difficult for accurate printing to be performed using a print head that includes multiple nozzles having different ejection port diameters, and for the durability of the peripheral portions of the heaters to be improved.
As another reason that it is difficult to prevent the occurrence of cavitation, there is a case wherein the lengths of peripheral flow paths at the ejection ports of the print head, in a direction in which ink is ejected from a substrate to the wall face of the ejection ports, are extended in order to increase the printing speed. When the flow paths are formed in the nozzles in this manner, resistance to the flow of ink at the nozzles may be reduced while ink is supplied; however, when the length of a flow path to an ejection port from the substrate is extended for a nozzle that enables the ejection of a small ink droplet, employment of the arrangement that permits the bubbles to communicate with the air is more difficult.
As one other problem, when ink is ejected, the ink is generally divided into a main droplet and trailing sub-droplets, called satellites, and when a print head is formed so that bubbles communicate with the air, controlling the direction of ejected satellites is difficult. Furthermore, in accordance with recent developments in the study of small droplet formation during ejection, it has been found that satellites form into a mist and, as a result, the quality of a printed image is adversely affected by the low accuracy with which the satellites land. Thus, it may be concluded that means for improving the accuracy with which satellites land is required.
However, when the nozzles formed for a print head are designed to avoid the occurrence of cavitation by permitting bubbles to communicate with the air, the shapes of the bubbles are not stable and increasing the accuracy with which satellites land is difficult. Moreover, for a print head wherein heaters are alternately arranged in a zigzag pattern, and nozzles are arranged to permit bubbles to connect with the air, the low accuracy with which satellites land is especially obvious for a nozzle whose distance from an ink supply port is comparatively large.